An exemplary wireless telecommunications system 1 is illustrated in FIG. 1 of the accompanying drawings, and includes a number of basestations 10a, 10b and 10c, which communicate over a radio frequency (RF) air interface with mobile terminals 20a, 20b, 20c, 20d, and 20e. The mobile terminals are sometimes referred to as “user equipment (UE)”. It will be appreciated that the number of basestations and mobile terminals is merely exemplary, and that the systems can include any appropriate number of such units.
In order for communication, that is, data transfer, to be successful between the basestations 10 and the mobile terminals 20, communications channels are allocated to respective mobile terminals. These channels, or “links”, can be allocated in the time domain and/or the frequency domain, as is well known. Communications from a basestation 10 to a mobile terminal 20 are known as downlink communications, and communications from the mobile terminal 20 to the basestation 10 are known as uplink communications. The communications channels allocated for such data transfer are known as the downlink (DL) and the uplink (UL) respectively.
In order to increase capacity and coverage, some wireless telecommunications systems make use of link adaptation and scheduling. In order to support such techniques, feedback information, such as channel state information (CSI) or channel quality indicator (CQI) information is required. Such feedback information has to be supplied frequently and accurately so that the allocated communication channels in the system can be maintained at the appropriate quality level. As is known, transmission mode adaptation and link adaptation are terms used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link (for example, the interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.). The process of link adaptation is a dynamic one and the signal and protocol parameters can change as the radio link conditions change.
For a wideband wireless system that supports a large number of mobile terminals, the bandwidth required for the signalling of CSI/CQI and other feedback information is very high, and represents a very significant burden on the uplink. Although this is true for both time-domain, and for time and frequency domain systems, it is the time and frequency domain system that suffers most, since the total resource is cut to much finer granularity and a higher number of CSI/CQI signals are required due to more small-size sub-units.
3GPP™ Release 5 (Third Generation Partnership Project) introduces the concepts of link adaptation and scheduling in HSDPA (High-Speed Downlink Packet Access) systems. The resource allocation and transport format and resource related information (TFRI) is carried by the High Speed Shared Control Channel (HS-SCCH). The HS-SCCH is two time slots ahead of the data transmission and is carried over the High Speed Downlink Shared Channel (HS-DSCH). Accordingly, scheduling and link adaptation is carried out at the same time, two time slots ahead of the data transmission. Every mobile terminal in the system performs channel quality reporting at regular intervals, so that such signalling is transmitted for each channel in the system on a continual basis.
In addition, it can be very difficult to support accurate feedback for systems using transmitter only precoding. Ideally CQI feedback (to support fast link adaptation) and the preferred transmit weights (on which to base the channel matrix and the covariance matrix of interference and noise), or the channel matrix and covariance matrix themselves, are required for accurate link adaptation.
It will, therefore, be appreciated that use of link adaptation and scheduling in wideband wireless telecommunications systems is desirable in order to increase capacity of the systems, but has the significant problem that channel quality reporting by the mobile terminals uses an undesirably high proportion of the available uplink bandwidth. It will also be readily appreciated that providing a technique whereby high quality feedback information can be supplied in advance of link adaptation is desirable.